The present invention relates to a thin-film transistor array to be used as a driver or switching circuit in printers, contact-type image sensors, display devices and the like.
Referring to FIG. 1, there is shown an example of a conventional thin-film transistor array.
In FIG. 1, Q.sub.1 to Q.sub.3 represent thin-film transistors which form a thin-film transistor array; V.sub.G1 to V.sub.G3 represent gate signals; V.sub.D1 to V.sub.D3, data signals; C.sub.L1 to C.sub.L3, load capacitances; V.sub.O1 to V.sub.O3, output signals; C.sub.GS1 to C.sub.GS3, gate-source capacitances; S, a source; D, a drain; G, a gate. Although only three transistors are shown in FIG. 1, it is to be understood that a large number of transistors are arranged in practical use.
The operation of each thin-film transistor in such a thin-film transistor array will be described with reference to an arbitrary thin-film transistor (for example, Q.sub.1).
An outline of the operation is as follows. When a thin-film transistor Q.sub.1 is turned on by a gate signal V.sub.G1, a load capacitance C.sub.L1 is charged by a data signal V.sub.D1. A resultant voltage across the load capacitance C.sub.L1 is used as an output signal V.sub.O1.
The detail of the operation will be described with reference to FIG. 2 which shows waveforms for explaining the operation of one thin-film transistor Q.sub.1 in FIG. 1. In FIG. 2, the diagrams (a), (b) and (c) show respective waveforms of the data signal V.sub.D1, gate signal V.sub.G1, and output signal V.sub.O1.
In the following, the operation is described with the passage of time.
(1) Operation from t.sub.1 to t.sub.2
The gate signal V.sub.G1 is turned to a high value V.sub.GG by a pulse b.sub.1, so that the thin-film transistor Q.sub.1 is switched on. At the same time, the data signal V.sub.D1 is turned to a high value V.sub.DD by a pulse a. Accordingly, the load capacitance C.sub.L1 is charged by the data signal having the value V.sub.DD, and a resultant voltage across the load capacitance C.sub.L1 is outputted as an output signal V.sub.O1.
The waveform of the output signal V.sub.O1 shown in FIG. 2(c) is drawn so as to jump by .DELTA.V at the time t.sub.1 relative to the preceding value. This is because when the gate signal V.sub.G1 rises to V.sub.GG, the rapid change of the voltage is transmitted to the source S through the gate-source capacitance C.sub.GS1 intrinsically existing between the gate G and source S to thereby increase a potential at the load capacitance C.sub.L1.
This is called a feed-through phenomenon and the changed voltage .DELTA.V is called a feed-through voltage. The value .DELTA.V is expressed by the following equation: ##EQU1##
As the charging of the load capacitance C.sub.L1 progresses, the value of the output signal V.sub.O1 increases. Because the difference between the output signal value V.sub.O1 and the data signal value V.sub.GG is reduced with the increase of the output signal value V.sub.O1, a current flowing through the thin-film transistor Q.sub.1 becomes small, so that the increase rate of the output signal V.sub.O1 becomes small. It is not long before the output signal V.sub.O1 reaches an output high-level peak V.sub.H.
This process forms a waveform c.sub.1 in FIG. 2(c). The output signal V.sub.O1 is kept at the output high-level peak V.sub.H until the gate signal V.sub.G1 falls at t.sub.2.
(2) Operation from t.sub.2 to t.sub.3
When the gate signal V.sub.G1 falls at the time t.sub.2, the rapid potential change is transmitted to the source S through the gate-source capacitance C.sub.GS1 to thereby decrease the potential at the load capacitance C.sub.L1, i.e., the output signal V.sub.O1. This decrease of the potential is also referred to as the feed-through voltage .DELTA.V, as described above.
A settled value after the decrease of the potential is an output high level V.sub.OH. Because the thin-film transistor Q.sub.1 is kept off until the gate signal V.sub.G1 is turned to the high value V.sub.GG again at t.sub.3, the output signal V.sub.O1 is kept at the output high level V.sub.OH. Some operation corresponding to the output high level is carried out in an external system using the value V.sub.OH.
(3) Operation from t.sub.3 to t.sub.4
When a pulse b.sub.2 of the gate signal V.sub.G1 rises at the time t.sub.3, the output signal V.sub.O1 increases instantaneously by the feed-through voltage .DELTA.V due to the feed-through phenomenon. Because the data signal V.sub.D1 is, however, zero in a period from t.sub.3 to t.sub.4, the load capacitance C.sub.L1 is discharged to reduce the output signal V.sub.O1 to zero rapidly. This process forms a waveform C.sub.2 in FIG. 2(c).
(4) Operation at and after t.sub.4
When the pulse b.sub.2 of the gate signal V.sub.G1 falls at the time t.sub.4, the output signal V.sub.O1 decreases by the feed-through voltage .DELTA.V due to the feed-through phenomenon, so that the output signal V.sub.O1 takes a minus peak. The minus peak is referred to as an output low-level peak V.sub.L.
Accordingly, relations between potentials around the thin-film transistor Q.sub.1 immediately after the gate signal V.sub.G1 becomes zero at t.sub.4 are as follows. The potential (0) at the gate G is positive relative to the potential (-.DELTA.V) at the source S. The potential (0 as a value of the data signal V.sub.D1) at the drain D is also positive relative to the potential at the source S.
It is, however, commonly known that the source S and drain D of the thin-film transistor are symmetric in structure, and the source S and drain D can serve interchangeably.
Therefore, a current continues to flow through the thin-film transistor Q even after t.sub.4, so that the output signal V.sub.O1 increases gradually from the output low-level peak V.sub.L. Ultimately, the voltage between the source S and drain D is settled to the threshold voltage V.sub.th of the thin-film transistor Q.sub.1, that is defined as a specific voltage level between the opposite ends (source S and drain D) of the thin-film transistor Q.sub.1 with a voltage below which level no current can flow through the thin-film transistor Q.sub.1. The settled value is referred to as an output low level V.sub.OL. In short, V.sub.OL =-V.sub.th. This process forms a waveform c.sub.3 in FIG. 2(c).
An operation to be conducted when the output signal V.sub.O1 is low, is carried out in an external system using the output low level V.sub.OL.
A description of such a thin-film transistor array is found in the literature by Malcolm J. Thompson and Hsing C. Tuan, entitled "Amorphous Si Electronic Devices and Their Applications", IEDM 86 pp. 192-195 (particularly, FIG. 8).
However, the following problems are caused by the feed-through phenomenon in the aforementioned conventional thin-film transistor arrays.
The first problem is that the output high level V.sub.OH is reduced by the feed-through voltage .DELTA.V from the given data signal value V.sub.D1 to a value often insufficient for a desired operation. To assure the desired operation, a voltage higher by the feed-through voltage .DELTA.V than the output high level V.sub.OH must be prepared as the data signal V.sub.D1.
The second problem is that the output low level V.sub.OL depends on the threshold voltage V.sub.th of the thin-film transistor which changes with the passage of time during the operation of the thin-film transistor. Accordingly, the output low level V.sub.OL cannot be stable. When the output low level V.sub.OL is not stable, for example the following disadvantage arises. This is, in the case where the thin-film transistor array is used in an image processing device or the like to attain a tonal feature, the potential difference between the output high level and low level is not stabilized, causing erroneous results.